Actuators have long been used to control the operation of other devices, such as flow-controlling valves. Such actuators have normally been hydraulically, pneumatically, or electrically motivated. Oftentimes back-up systems were desirable, to permit actuation in the event of power failures, or the like. Means have previously been developed to permit selective engagement and disengagement of such back-up systems. An example of such development is illustrated in U.S. Pat. No. 3,677,108. Although the latter mentioned development represented a substantial advance over the prior art, it has proved desirable in some applications to ensure positive engagement of said back-up system. Positive disengagement may also be desired. It is to the solution of such problems that this invention is directed.